Apparatus for producing charged liquid particles



Nov. 17, 1964 v. E. KROHN, JR 3,157,819

APPARATUS FOR PRODUCING CHARGED LIQUID PARTICLES Filed Nov. 22. 1960 \4 I \5 VARIABLE VAN/ABLE VO LTAGE PRESSURE. 50 U RCE CO NTROL .9- 2 HEATER CONTROL IIIIIIIIIIIIIIIIIIIII. ai -4 1 M 'IIIIIIIIIIIIIIIIIIII" 2.

V/CTOR E. /e0H/v, IQ.

INVENTOR.

BY WM A 770 /2NE Y5 United States Patent This invention relates to charged particle generating apparatus and more particularly to an improved arrangement for providing particles and ions bearing an electrical charge.

Of the various types of propulsion systems for space vehicles which are known or are being developed, the type which derives propulsive thrust from electrostatically accelerated particles is of special interest for artists Patented Nov. 17, 1%64 ice It is another object of this invention to produce a charged particle source capable of generating charged particles of variable size as desired.

It is a further object of this invention to provide apparatus for producing charged particles having a vari-,

ably controllable charge-to-mass ratio.

It is an additional object of this invention to provide apparatus capable of generating monatomic ions from a substantially point source.

Another object of the invention is the provision of charged particle generating apparatus which is lighter in weight, easier to fabricate and more reliable in operation space vehicle propulsion in cases where the generated thrust need not be large but is tobe applied continuously over a relatively long period of time. In such systems, the thrust developedis derived from a change of momenturn of the charged particles as they are accelerated through an electric field of relatively high intensity.

Space vehicle propulsion systems are known which depend upon charged ions as a propellant. It may be noted that the term ions as used herein refers to essentially monatomic charge carriers while the term charged particles is reserved for particles containing hundreds or thousands of atoms and carrying one or more elementary charges. It has, been found that certain advantages accrue from the use of the larger charged particles in an electrostatic propulsion system rather than when ions are employed. Because the ratio of mass to charge is much greater for particles than for ions, the spacing of the electrodes of the accelerating structure may be extended when charged particles are employed, thus minimizing the mechanical difiiculties of providing appropriate electrode spacing in the accelerating structure.

Another problem which/is considerably simplified by the use of charged particles for the propellant is that of neutralization of the expelled particle beam. It is considered essential that the beam be electrically neutralized as it leaves the space vehicle in order to maintain the vehicle electrically neutral and to maximize the thrust generated by the accelerating structure. This problem has not yet been resolved entirely although a number of attempts at its solution have been proposed. The problem is rendered less acute by the use of a narrow beam configuration. Such a configuration permits the beam to diverge as it leaves the vehicle, thus reducing the attendant space charge density. Where the beam diverges in this fashion, space charge neutralization close to the vehicle is rendered unnecessary. The use of charged particles, rather than ions, as the propellant renders the narrow beam configuration more feasible by providing a much greater thrust from a narrow beam.

One thrust-producing arrangement which may be adapted to function as a propulsion system for a space vehicle is disclosed in United States Patent 2,880,337 of D. B. Langmuir et a]. In this arrangement particles are presented at one end of an accelerating structure, after which they are given a particular charge by bombardment with electrons or ions so that they may respond to the electric field maintained by the accelerating structure. Furthermore, the particles originate from a source of relatively broad extent, thus requiring a greater degree of beam focusing in order to obtain a particle beam of suitable diameter.

It is an object of this invention to produce an improved charged particle generator suitable for inclusion in a space vehicle propulsion system.

than other apparatus heretofore developed for this purpose.

Briefly, the invention involves the application of a high electric field to a supply of liquid metal presented at a narrow orifice under pressure. Under various conditions of pressure and applied electric field, it has been found that minute droplets of the liquid metal varying in size over a relatively wide range are emitted from the orifice. Thus an arrangement is provided for producing large numbers of charged particles from what is essen-' tially a point source. Furthermore, the size of the droplets is controllable within broad limits.

it will be appreciated that the invention produces droplets which are already electrostatically charged, rather than requiring that the charge be added after the particles are generated. Furthermore, it has been found that a copious supply of ions is produced along with the droplets and that the proportion of charged droplets to ions generated depends on the magnitude of the electric field applied and other controllable factors. Thus the invention is also suitable as a point source of ions and may find application in such devices as ion microscopes, in addition to being useful for charged particle propulsion systems for space vehicles.

In accordance with an aspect of the invention, specially shaped electrodes may be positioned near the orifice from which the liquid metal emerges to provide particular concentrations and configurations of the applied electric field in accordance with the desired properties of the particles being generated.

The invention may be better understood from a consideration of the following detailed description taken in conjunction with the drawing, in which:

FIG. 1 represents one specific embodiment of the invention and shows the charged particle generating structure thereof in perspective View;

FIG. 2 is a side view, in cross section, of the interior portion of the charged particle generating structure of FIG. 1; and

FIG. 3 is an enlarged cross-sectional view of a portion of the structure of FIG. 2 modified in accordance with one specific embodiment of the invention.

In FIG. 1, which represents a particular arrangement of the invention, a grounded cylindrical metal shield 1. is shown encompassing a narrower cylinder 2. Positioned on the cylindrical shield 1 are a pair of heater coils 3 which are connected to a heater control device 4. Attached to the cylindrical shield 1 at right angles is a stub section 5 terminating in a planar electrode 6 having a central aperture. therein. Within the section 5 and extending through the aperture of the electrode 6 is a right angled attachment to the cylinder 2 which is shown in greater detail in FIG. 2. Connected to the right angle section 7 is a narrow tube 8. A rod 9 of suitable insulating materialis attached to a threaded portion at the lower end of the cylinder 2 for positioning the cylinder within the shield 1.

In FIG. 1 the cylinder 2 is shown connected to a pipe 11 which is in turn attached to a pressure control device 12. The pipe 11 is electrically connected to a variable voltage source 13 by a lead 14. In this arrangement the pipe 11 is of metal and thus maintains the cylinder 2 at a potential provided by the variable voltage source 13.

In one specific embodiment of the invention, the tube 8 may have an inside radius of 62 microns at the tip and may be secured to a tube 2 having an inside diameter of 0.44 inch. The end of the tube from which the charged particles are emitted is arranged to project through a round aperture in a grounded planar electrode 6 a distance suitable to achieve the desired configuration of electric field. The field is related to some extent to the aperture diameter and the distance from the ground electrode plane to the tip of the tube. However, neither dimension is critical and the effect of varying these dimensions is substantially negligible over a considerable range of operating conditions. In the describedarrangement of the invention the tip of the tube may extend 5 inch from the plane of the ground electrode through an aperture approximately A inch in diameter.

In the operation of the structure of FIGS. 1 and 2, the cylinder 2 is filled with a metal of low melting point such as Woods metal. Current in the heater circuit comprising the heater control source 4 and the heater coils 3 maintains this metal in its molten state. Pressure is applied to the metal within the cylinder 2 from the pressure control device 12 through the pipe 11. Simultaneously the potential of the structure comprising the pipe 11, cylinder 2 and the tube 8 is controlled at a selected elevated potential by the voltage source 13. Since the planar electrode 6 is grounded, the applied potential difference is developed between the electrode 6 and the tip of the tube 8, thus establishing a high electric field in this region. Because of the configuration of the depicted electrodes 6 and 8, the electric field at the tip of the tube 8 causes particles in the form of .charged droplets to be emitted therefrom. With this arrangement, the size of the emitted droplets and their respective ratios of chargeto-mass can be controlled within broad limits by selectively varying the potential from the voltage source 13 and the pressure supplied to the reservoir of the cylinder 2 from the pressure control device 12. By applying a sufiiciently high potential, the character of the emitted particles can be changed so that they are essentially all ions rather than charged metal droplets if desired.

In the depicted embodiment when the voltage is increased while the pressure behind the liquid metal is maintained constant, a point is reached where the droplet generation commences suddenly. The voltage at the tip of the tube 8 falls as a result of the voltage drop across a limiting series resistor (not shown) but the droplet generation continues. The current represented by the charge transmitted by the generated droplets can then be increased considerably with a very slight increase in voltage.

The droplets vary somewhat in size under fixed generating conditions and conform to a particular distribution in number with respect to individual size. As the current is increased the individual charge on the larger droplets tends to increase and their number decreases relative to the number of smaller droplets. The charge-to-mass ratio of the smaller droplets remains essentially unchanged. If the pressure is reduced with the voltage being raised to restore droplet generation, the same trend is noticed, that is, an increase in the individual charge of the large droplets and a reduction in their number.

The particular properties of a droplet are determined from the amplitude and time duration of the pulse induced as the droplet passes through a cylinder which may be 7 millimeters long and .5 millimeter in diameter located about 5 inches from the source. These induced pulses are displayed on an oscilloscope and photographed. Droplets with radii from 0.05 to 0.75 micron (500 to 7500 Angstroms) have been observed. The 0.05 micron droplets are the smallest which are distinguishable from noise with the detecting apparatus employed and have a charge of 4.5 coulombs, a charge-to-mass ratio of 80 4 coulombs/kilogram and an electric field at the surface of 1.6 X 10 volts/meter.

For given source conditions, droplets of a fixed mass have a distribution in charge over a range of :20% when conditions are stable and the current is high. The particular droplet mass distribution with size is ditficult to ascertain, however, because the peak in this distribution is believed to fall below the limit of detection for the detecting apparatus employed. However, extrapolation to droplets below the limit of detection leads to extremely high charge-to-mass ratios. Thus it is seen that the structure of the invention permits the generation of charged particles having a size which is controllable over a considerable range. Also the invention is capable of varying the charge carried by the droplets within certain limits.

Moreover it has been found that as the applied voltage is increased the droplets are no longer stable as such and the material is emitted essentially as ions. Accordingly, the invention advantageously provides a point source of charged particles controllable in size which may also be used as a point source of ions where such is desired.

FIG. 3 depicts a particular arrangement for providing a predetermined field configuration at the particle source and shows a slender rod 20 haveing a ball 21 at the end thereof positioned within the tube 8 of FIG. 2. The improved performance of this specificarrangement in the production of charged particles in accordance with the invention is attributed to the somewhat different configuration of electric field which is believed to result because of the shape of the electrode 20 and also because of the fact that the liquid metal has a tendency to be drawn out on the ball 21 from which the charged droplets are emitted. The electrode 20 is attached to the inside of the tube 8 at a distance from the tip thereof such that it does not interfere with the even flow of the liquid metal through the space between the electrode 20 and the tube 8.

In a preferred embodiment of the invention incorporating the arrangement of FIG. 3, the tube 3 was constructed of stainless steel having dimensions of 0.007 inch inside diameter and 0.013 inch outside diameter. The electrode 20 comprises a rod of 0.006 inch diameter which tapers to 0.003 inch minimum diameter. The ball 21 was of a diameter of 0.007 inch and a clearance of 0.002 inch was provided between the ball 21 and the tip of the tube 8. A suitable procedure for forming the ball 21 on the end of the rod 20 is to etch the rod to a point and then to heat the point with an electric discharge so as to melt only the end thereof.

Although specific arrangements of the invention have been mentioned and described above in order to illustrate the operation thereof, it will be appreciated that the invention is not to be restricted to these arrangements. Accordingly, any and all modifications, variations or equivalent arrangements falling within the scope of the annexed claims should be considered to be a part of the invention.

What is claimed is:

1. Apparatus for generating a stream of charged particles having a controllable charge-to-mass ratio comprising a container having a nopening therein, a supply of liquid metal within the opening, means for applying a variable pressure to the container, a grounded metal shield surrounding the container and having an aperture opposite said opening, and a variable voltage source connected to the container for establishing a variable electric field at said opening.

2. Apparatus in accordance with claim 1 wherein said container includes a tube-like sect-ion extending outwardly therefrom through said aperture and said shield includes a planar electrode containing said aperture and mounted at right angles to the longitudinal axis of said tube-like section.

3. Apparatus in accordance with claim 2 wherein said tubelike section includes an electrode connected thereto.

5 for developing a predtermined electric field configuration at the tube opening.

4. Apparatus in accordance with claim 3 wherein the electrode comprises a rod mounted within the tube-like section.

5. Apparatus in accordance with claim 4 wherein said rod is centrally positioned within said tube-like section.

6. Apparatus in accordance with claim 5 wherein said rod includes a ball-like portion on the tip thereof extending slightly from the end of the tube-like section.

7. Apparatus for generating a stream of ions from a supply of liquid metal comprising a container having a narrow opening therein, a supply of liquid metal within the container, heating means for maintaining the metal in a liquid state, a grounded planar electrode having an aperture disposed about said opening, means for maintaining a predetermined pressure on said container, and means for applying a selected voltage to the liquid metal in said container.

8. A source of charged particles comprising a supply of liquid metal, a hollow cylindrical container for said liquid metal having a section disposed outwardly near the bottom of said container, a tube having one end attached to said section, a shield surrounding said container, heater windings positioned on said shield, a heater control source connected to the heater windings for maintaining the metal in a liquid state, a planar electrode mounted at substantially right angles to the longitudinal axis of the tube and having an aperture through which the tip of the tube extends, variable pressure control means for maintaining a selected pressure on the metal in the container, and means for establishing a variable potential difference between the planar electrode and the tip of the tube.

9. A charged particle source in accordance with claim 8 wherein said tube is of stainless steel and has an inside diameter of 0.007 inch and an outside diameter of 0.013 inch. I

10. A charged particle source in accordance with claim 9 further including a metal electrode centrally positioned within said tube and having a 0.007 inch ball on the end thereof 0.002 inch outside the tip of the tube.

ll. A charged particle source in accordance with claim 8 wherein said tube has an inside radius of 62 microns at its tip and extends through said aperture a distance of A inch.

12. Apparatus for generating a stream of charged metal particles having a selectable charge-to-mass ratio within a panticular range comprising a vessel containing a supply of liquid metal, a planar electrode adjacent said vessel and connected to a reference potential, a tube-like extension connected to said vessel and terminating in a hollow rod extending through an aperture in said planar electrode, and means for emitting particles of the liquid metal from the tip of said hollow rod and for controlling the chargeto-rnass ratio for said particles comprising variable pressure control means and variable voltage source means connected to said vessel.

References Cited in the file of this patent UNITED STATES PATENTS Latour et a1 May 23, 1950 

1. APPARATUS FOR GENERATING A STREAM OF CHARGED PARTICLES HAVING A CONTROLLABLE CHARGE-TO-MASS RATIO COMPRISING A CONTAINER HAVING AN OPENING THEREIN, A SUPPLY OF LIQUID METAL WITHIN THE OPENING, MEANS FOR APPLYING A VARIABLE PRESSURE TO THE CONTAINER, A GROUNDED METAL SHIELD SURROUNDING THE CONTAINER AND HAVING AN APERTURE OP- 